Z. Z. Li

Evolution of the pseudogap from Fermi arcs to the nodal liquid

The pseudogap phase in the cuprates is a most unusual state of matter: it is a metal, but its Fermi surface is broken up into disconnected segments known as Fermi arcs. Using angle resolved photoemission spectroscopy, we show that the anisotropy of the pseudogap in momentum space and the resulting arcs depend only on the ratio T/T*(x), where T*(x) is the temperature below which the pseudogap first develops at a given hole doping x. In particular, the arcs collapse linearly with T/T* and extrapolate to zero extent as T goes to 0. This suggests that the T = 0 pseudogap state is a nodal liquid, a strange metallic state whose gapless excitations are located only at points in momentum space, just as in a d-wave superconductor.

Protected nodes and the collapse of Fermi arcs in high-T{c} cuprate superconductors.

Angle resolved photoemission on underdoped Bi2Sr2CaCu2O8 reveals that the magnitude and d-wave anisotropy of the superconducting state energy gap are independent of temperature all the way up to T{c}. This lack of T variation of the entire k-dependent gap is in marked contrast to mean field theory. At T{c} the point nodes of the d-wave gap abruptly expand into finite length Fermi arcs. This change occurs within the width of the resistive transition, and thus the Fermi arcs are not simply thermally broadened nodes but rather a unique signature of the pseudogap phase.

Absence of a loss of in-plane infrared spectral weight in the pseudogap regime of Bi(2)Sr(2)CaCu(2)O(8+delta).

The ab-plane reflectance of Bi2Sr2CaCu2O(8+d) thin films was measured in the 30-25000 cm^(-1) range for one underdoped (Tc = 70 K), and one overdoped sample (Tc = 63 K) as a function of temperature (10-300 K). We find qualitatively similar behaviors in the temperature dependence of the normal-state infrared response of both samples. Above Tc, the effective spectral weight, obtained from the integrated conductivity, does not decrease when T decreases, so that no opening of an optical pseudogap is seen. We suggest that these are consequences of the pseudogap opening first in the k=(0, pi) direction, according to ARPES, and of the in-plane infrared conductivity being mostly sensitive to the k=(pi, pi) direction.

Crossover from coherent to incoherent electronic excitations in the normal state of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+delta}.

Angle resolved photoemission spectroscopy (ARPES) and resistivity measurements are used to explore the overdoped region of the high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+delta). We find evidence for a new crossover line in the phase diagram between a coherent metal phase, for lower temperatures and higher doping, and an incoherent metal phase, for higher temperatures and lower doping. The former is characterized by two well-defined spectral peaks in ARPES due to coherent bilayer splitting and superlinear behavior in the resistivity, whereas the latter is characterized by a single broad spectral feature in ARPES and a linear temperature dependence of the resistivity.

Observation of a d -wave nodal liquid in highly underdoped Bi 2 Sr 2 CaCu 2 O 8+ δ

High-temperature superconductivity in the cuprates arises when charge carriers are added to an insulator. Between these states lies the so-called nodal liquid at low temperature. Photoemission spectroscopy suggests that superconductivity evolves smoothly from this nodal-liquid state.

Nondispersive Fermi Arcs and the Absence of Charge Ordering in the Pseudogap Phase of Bi2Sr2CaCu2O8+δ

The autocorrelation of angle resolved photoemission data from the high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) shows distinct peaks in momentum space which disperse with binding energy in the superconducting state, but not in the pseudogap phase. Although it is tempting to attribute a nondispersive behavior in momentum space to charge ordering, a deconstruction of the autocorrelation reveals that the nondispersive peaks arise from the tips of the Fermi arcs, which themselves do not change with binding energy.

Oxygen-induced superconducting, metallic or insulating behaviour in as-grown epitaxial Bi2Sr2CuOx thin films

Abstract Epitaxial Bi2Sr2CuOx thin films were prepared in situ by reactive RF magnetron sputtering. Superconducting films with Tc (R=0) up to 18 K were obtained which correspond to what we call an optimal oxygen content. It is the highest Tc reported for films of the 2201 phase. In the case of films made under the same sputtering conditions, the superconducting critical temperature value decreases and superconductivity disappears when the oxygen content in the films is decreased or increased during the cooling process. With reference to the optimal oxygen content, overdoped films tend to display a normal metal behaviour and underdoped films tend to display a semiconductor-like behaviour at low temperature.

Evolution of the resistivity of single-layer Bi2Sr1.6La0.4CuOy thin films with doping and phase diagram

Abstract The temperature dependence of the in-plane resistivity ρ ( T ) is measured on epitaxial c -axis oriented single-layer Bi 2 Sr 1.6 La 0.4 CuO y thin films at various oxygen concentrations. By successive annealing treatments, the oxygen content of the same film is changed from maximally overdoped to strongly underdoped non-superconducting state, passing through the optimal state with T c max =30 K . The underdoped states show a downturn of the resistivity from the high T -linear behavior below a characteristic temperature T *, signature of the pseudogap effect. T * appears near optimally doped state and increases sharply with decreasing carrier concentration. Two other characteristic temperatures are observed in ρ ( T ) for underdoped states: the temperature T I of the inflection point in ρ ( T ) ( T I ∼0.5 T * ) and the temperature T M corresponding to the onset of localization effects. A phase diagram T versus doping is established.

In-plane electrodynamics of the superconductivity inBi2Sr2CaCu2O8+δ: Energy scales and spectral weight distribution

The in-plane infrared and visible (3 meV–3 eV) reflectivity of Bi2Sr2CaCu2O8+� (Bi-2212) thin films is measured between 300 K and 10 K for different doping levels with unprecedented accuracy. The optical conductivity is derived through an accurate fitting procedure. We study the transfer of spectral weight from finite energy into the superfluid as the system becomes superconducting. In the over-doped regime, the superfluid develops at the expense of states lying below 60 meV, a conventional energy of the order of a few times the superconducting gap. In the underdoped regime, spectral weight is removed from up to 2 eV, far beyond any conventional scale. The intraband spectral weight change between the normal and superconducting state, if analyzed in terms of a change of kinetic energy is � 1 meV. Compared to the condensation energy, this figure addresses the issue of a kinetic energy driven mechanism.

Change of Fermi-surface topology in Bi 2 Sr 2 CaCu 2 O 8 + δ with doping

We report the observation of a change in Fermi-surface topology of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} with doping. By collecting high-statistics angle-resolved photoemission spectroscopy data from moderately and highly overdoped samples and dividing the data by the Fermi function, we answer a long-standing question about the Fermi-surface shape of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} close to the ({pi},0) point. For moderately overdoped samples (T{sub c}=80 K) we find that both the bonding and antibonding sheets of the Fermi surface are hole like, but for a doping level corresponding to T{sub c}=55 K we find that the antibonding sheet becomes electron like. Similar observations of a topology change were observed in La{sub 2-x}Sr{sub x}CuO{sub 4} and in Bi{sub 2}Sr{sub 2}CuO{sub 6+{delta}}. On the other hand, whereas this critical doping value in single-layer materials corresponds to a T{sub c} near zero, it occurs at a smaller doping value in the double-layer case where T{sub c} is still quite high (the difference in doping levels is due to the bilayer splitting). This argues against a van Hove singularity scenario for cuprate superconductivity.